linux-kernel - Re: [PATCH v3 3/4] cxl/mem: Do not rely on device_add() side effects for dev_set

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Message-ID: <20210331161759.GL1463678@nvidia.com>
Date:   Wed, 31 Mar 2021 13:17:59 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     linux-cxl@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Vishal L Verma <vishal.l.verma@...el.com>,
        "Weiny, Ira" <ira.weiny@...el.com>,
        "Schofield, Alison" <alison.schofield@...el.com>
Subject: Re: [PATCH v3 3/4] cxl/mem: Do not rely on device_add() side effects
 for dev_set_name() failures

On Wed, Mar 31, 2021 at 09:04:32AM -0700, Dan Williams wrote:
> On Wed, Mar 31, 2021 at 6:10 AM Jason Gunthorpe <jgg@...dia.com> wrote:
> >
> > On Tue, Mar 30, 2021 at 04:36:42PM -0700, Dan Williams wrote:
> > > +static int cxl_mem_add_memdev(struct cxl_mem *cxlm)
> > > +{
> > > +     struct cxl_memdev *cxlmd;
> > > +     struct device *dev;
> > > +     struct cdev *cdev;
> > > +     int rc;
> > > +
> > > +     cxlmd = cxl_memdev_alloc(cxlm);
> > > +     if (IS_ERR(cxlmd))
> > > +             return PTR_ERR(cxlmd);
> > > +
> > > +     dev = &cxlmd->dev;
> > > +     rc = dev_set_name(dev, "mem%d", cxlmd->id);
> > > +     if (rc)
> > > +             goto err;
> > >
> > > +     cdev = &cxlmd->cdev;
> > >       cxl_memdev_activate(cxlmd, cxlm);
> > >       rc = cdev_device_add(cdev, dev);
> > >       if (rc)
> > > -             goto err_add;
> > > +             goto err;
> >
> > It might read nicer to have the error unwind here just call cxl_memdev_unregister()
> 
> Perhaps, but I don't think cdev_del() and device_del() are prepared to
> deal with an object that was not successfully added.

Oh, probably not, yuk yuk yuk.

Ideally cdev_device_add should not fail in a way that allows an open,
I think that is just an artifact of it being composed of smaller
functions..

For instance if we replace the kobj_map with xarray then we can
use xa_reserve and xa_store to avoid this condition.

This actually looks like a good fit because the dev_t has pretty
"lumpy" allocations and this isn't really performance sensitive.

A clever person could then make the dev_t self allocating and solve
another pain point with this interface. Hum..

Jason